Embodiments of the present invention relate to thin film batteries and their fabrication and packaging.
Thin film batteries are used in applications that require a small battery with a high energy density such as, for example, portable electronics, medical devices and space systems. A typical thin film battery typically comprises a substrate having one or more battery component films which cooperate to store electrical charge and generate a voltage. The battery component films include an electrolyte sandwiched between electrodes. Some of the battery component films can be metal-containing films which are composed of elemental metal, metal oxide or other metal-containing compounds. For example, elemental metal films can be used as current collectors to receive or provide electrons, such as for example, cathode and anode current collectors. Metal oxide or metal-containing compounds are useful as battery component films such as the cathode or electrolyte. The battery component films are thinner than conventional batteries, for example, the films can have thicknesses of less than 1000 microns, or even less than 100 microns. This allows thin film batteries to have thicknesses which are much smaller than the thickness of conventional batteries. The battery component films are often formed by processes such as physical and chemical vapor deposition (PVD or CVD), oxidation, nitridation, and electroplating processes. These batteries can either be used individually or stacked together to provide more power or energy.
In the thin film fabrication processes, the battery component films are sometimes heat treated to anneal, re-crystallize, or reduce lattice defects from the deposited film. For example, elemental metal films are heat treated to reduce lattice defects and provide better conductivity. Metal oxide films are also sometimes heated in air to anneal and/or obtain better crystalline properties. As an example, a cathode comprising a metal oxide electrode, such as for example lithium cobalt oxide in a lithium thin film battery, provides better electrical properties when annealed in oxygen-containing environment at temperatures ranging from 300 to 700° C. It is believed that the electrical characteristics of the annealed cathode is related to its oxygen content in its crystallographic structure, and when annealed, the cathode can allow the battery capacity to reach as high as its theoretical value for a given thickness and area. Still further, the lithium oxide cathode film can be deposited as an amorphous or microcrystalline film or as a stack of sequentially deposited thin amorphous films to form a thicker cathode, and thereafter, crystallized by heating the amorphous film or stack of films. Increasing the thickness of a lithium oxide cathode film increases the energy density of the battery as the thicker cathode film provides greater charge retention and faster charging and discharging, as for example, taught in commonly assigned U.S. patent application Ser. No. 11/007,362 entitled “THIN FILM BATTERY AND METHOD OF MANUFACTURE” which is incorporated by reference herein in its entirety.
However, the heat treatment process needed for a battery component film is often conducted after the deposition of underlying battery component films, and can result in thermal degradation or even oxidation of the underlying films. For example, in lithium batteries, a cathode current collector of elemental metal is often deposited below a cathode of metal oxide. Heat treatment of the overlying metal oxide cathode can result in oxidation of the underlying elemental metal of the current collector. For example, current collectors made from metals such as aluminum or copper can form an oxidized thin film layer of aluminum oxide or copper oxide, respectively, which increases the electrical resistance at the interface of the current collector and the overlying anode or cathode electrode to reduce overall battery efficiency.
To prevent or reduce the effects of thermal or oxidation degradation, noble metals such as platinum or gold, are often used as the current collectors. However, these metals are expensive and can substantially increase the cost of the resultant battery. In addition, metals such as platinum often exhibit poor adhesion to the underlying substrate requiring deposition of an adhesion layer between the substrate and platinum to prevent the metal from peeling. The additional deposition steps required for forming the adhesion layers further increase processing time and cost.
For reasons including these and other deficiencies, and despite the development of various battery structures, and deposition and heat treatment processes for thin film batteries, further improvements in battery structures and heat treatment methods are continuously being sought.